Key signature & chords curiosity
I got my hands on a sample page of the sheet music for Ode to Billy Joe. Unfortunately it's not licensed in the US, so I can't buy and download the whole thing. Oh well... :|
Anyway, the chords listed do match what I found on an internet chord sheet. Amazing! The key signature on the sheet music indicates key of D... 2 sharps, F# & C#. But the chords... I cannot for the life of me see how they belong to the key of D. They are D9 D7 G7 C7 Am7.
I've got chords in D as
D Em F#m G A Bm C#dim
DMaj7 Em7 F#m7 GMaj7 A7 Bm7 C#m7b5
How is it possible to have chords that don't seem to fit a key signature? Or is something else going on I don't know about yet?
It is difficult to answer when one does not understand the question.
Fair enough question and I think Tom or Fretsource will give you a more thorough answer, but the short one is to think about Ode to Billy Joe as a variation on the blues, but one that includes two blue notes (which are a prominent part of using the minor pentatonic for soloing) as part of the chord extensions.
In the key of D, you'd use the D minor pentatonic to play a blues-y solo over the chords. The two blue notes in the D minor pentatonic would be F (minor third) and C (flat seventh). Adding C to D (while keeping the F# of the key signature) gives you D7. Adding an E to that gives you D9. Adding F to G gives you G7. Replacing the C for the C# in A gives you Am. The Bb of the C7 makes a nice leading tone down to that A note.
I know you know that there is no hard and fast rule that says you have to use only the notes of any given key, or the diatonic chords for that matter. Sometimes it's all a matter of how the chords move from one to another and in this case the overall blues feel makes the chords work.
Again, this is the quick two-bit explanation. But I hope it helps until someone else makes it more easily understood. And my apologies in advance if they also tell you I'm totally off-base!
I found a copy of the sheet music in a book I have from 1970 - the New York Times Great Songs of the 60s. In that book it's got a key signature of C, and the chords are marked A7, Em7, D7, and G7.
If we consider the D7 and D9 in your arrangement as one chord (a dominant D chord), we've each got three dominant 7ths around the circle of fifths, and one m7 chord. The chords in your version are a perfect 5th higher than the ones in mine, but we've essentially got the same harmony. But if my key signature was a perfect 5th lower than yours, it would be G, and it's not.
So I'm going to break your post down into three questions to try to shed some light on what's going on...
Q1: Why do we use chords outside a key?
A1: Because it sounds good. Songwriters are free to use whatever they like, whether the chords are native to the key or not.
Q2: Why would we favor one key signature over another?
A2: Composers, arrangers, and publishers make their decisions based on making the standard notation easy to follow. Sometimes that's easy, but sometimes different elements of the song don't agree, and they have to decide what's most important to see at a glance. In this case, the melody is in a minor key (A minor in my version, D minor in yours). The publisher of my version treated the melody as the most important element, and the key of Am has no sharps or flats in the key signature - it's the same as the key of C. Your publisher treated the harmony as the most important element, and the main chords are based on D, G, and A - the I, IV, and V in the key of D. Even though it's in a minor key, it's using I7, so they used a major I from the chord progression as the key signature.
Generally speaking, conflicts between the melody and the chords that force publishers to make choices happen in a couple of circumstances: blues, where the usual preference is to use the key signature of the chord progression and use accidentals for the melody line, and modal music. The melody here is in D Dorian, so this song has two reasons publishers might make different choices.
Q3: How does this particular progression work?
A3: As David said, you can think of it as a blues. A Dm blues progression would usually use Dm-Gm-A7, but blues will often substitute a dominant 7 for any (or all) of the chords. Your D7 and D9 are dominant substitutions for Dm; your G7 stands in for Gm.
Am7 is used instead of A7 because the melody is in D Dorian - it's got a #6 (B natural). Every place the Am7 is used, the melody has just two notes - in your version, they'd be B and D. If we used A7, the C# in the chord and the D in the melody would create a very dissonant sound, and we can avoid that by lowering the third in the chord. Composers use tweaks like that all the time. In classical composition, a composer might highlight the conflict between a chord progression and a modal scale by using A7 sometimes, and Am7 at other times (a technique called harmonic inflection), but Bobbie Gentry just stuck with the one substitution throughout.
The C7 is kind of interesting. It only happens in one measure right at the end of your progression. And right at that spot in the melody, she's leaving the D Dorian scale she's used throughout, using F# in that measure and the one before it.
In the measure before it, you've got a D7 chord, and the F# in the melody creates a nice harmony - up to this point, there's been a conflict between the melody and the harmony whenever D7 is used, and that gives it a bluesy sound. So she's resolving that by changing the melody line slightly - a melodic inflection. In that measure, six of the eight eighth notes are chord tones.
But she's going to return to the Dorian scale in the next verse, and she wants to highlight that shift back. Compared to the major scale, the Dorian is 1-2-b3-4-5-6-b7, so she uses a bVII chord - that results in a I7-bVII7-I7 in the chords, using the Dorian scale notes as the chord roots and dominant 7 substitutions from the blues. But she's also retaining the F# in the melody line from the previous measure - creating a bluesy tension against the G7 chord. So you can also look at this measure as a temporary shift to the key of G, using I7. She might have thought of it either way - or not thought about it at all; it sounded good to her, so it was right (see A1). Theorists come in after the fact and try to say why it's right, and there can often be more than one way to interpret it.
It makes it sort of a fusion between folk and blues, and it's a pretty effective way to do it.
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I see NoteBoat got in first with lots of detailed info - but I'll post anyway and you can think of it as a footnote :D
It's been years since I've heard that song and I can't recall it well but I'm inclined to agree with David. The chords have been chosen to fit the bluesy melody. If you look at those chords, (D9 D7 G7 C7 Am7) you can see that no key signature will fit exactly as it would need Fsharp (D9, D7 ) AND F natural (G7) as well as Bflat (C7) AND B natural (G7).
So they've gone for the key of D because the tonal centre is D and the tonal centre chord is D major. That's enough to justify saying it's in the key of D major, even if it uses outside chords.
This is often the case with blues and modal music. They're not designed to fit nicely within the major/minor key system, but in sheet music they have to choose a key signature that makes sense and one way to make sense (there could be others) is to use the key signature associated with the tonal centre, which, being D major in this case, is the 2 sharp key signature.
Fantastic guys, thanks. :D The sad fact is that I wouldn't have gotten explanations like this either of the times I took lessons. At any rate, after reading the explanations, it really does make sense. There's actually a lot going on for such a "simple" song. Not only is the story mysterious and haunting, the music adds to it. For one of her first songs, Bobbie Gentry had it going on. Whether, as Tom said, she realized it or not. It just sounds good.
It is difficult to answer when one does not understand the question.
This was a great question and thanks to all who posted. Made for a really informative read.